The Reagan administration, faced with a solid wall of opposition from labor and congressional Democrats, retreated yesterday until after the election from proposing federal rules that would have expanded the hours and jobs that 14- and 15-year-olds can work.

It was the second time in recent weeks that the administration has pulled back on controversial Labor Department proposals that might have become a weapon for Democrats in the fall campaign. White House officials recently decided to scrap another set of rules that would have shrunk the scope of federal affirmative action enforcement.

In the case of the child labor laws, Labor Department officials insisted yesterday that there was no White House pressure to hold off until after the November elections.

The administration's retreat came just as the House labor standards subcommittee, chaired by Rep. George Miller (D-Calif.), was about to begin a second round of hearings on the administration's proposals, unveiled July 16. Miller has since introduced a resolution seeking to stop the proposals. He now has 95 co-sponsors, and Sen. Edward M. Kennedy (D-Mass.) has offered a similar measure in the Senate.

Robert B. Collyer, deputy undersecretary of labor for employment standards, said in a letter to Miller that the administration was extending the comment period for six months and would submit "new" rules later. An aide said it was not clear how much the rules would be revised. Collyer insisted that the administration is "committed . . . to the protection of teen-age workers first entering the labor force . . . ."

In response to questions, Collyer said through a spokesman he made the decision himself and it was "not the same situation" as the affirmative action rules, which reportedly were scrapped at the behest of presidential aides. A White House official who asked not to be identified confirmed his account.

"They were never able to show this was good for kids," said Miller. "Clearly, it is a victory that will force the administration to determine what is for the benefit of young children and not just what will improve the profits of the amusement (park) industry," one of the few to speak up last week in favor of the administration's proposals.

The rules were widely denounced at a hearing by labor, teachers, parents and child development groups, which said the changes were unnecessary at a time of high teen and adult unemployment and would contribute to academic troubles and delinquency among teens.

The administration proposals would modify the Fair Labor Standards Act of 1938, which permits young people under 16 to work under circumstances that don't interfere with their education and their "health and well-being." The rules would allow 14- and 15-year-olds to work as late as 9 p.m. on school nights instead of the current 7 p.m. cutoff, and as late as 10 p.m. on nights before no-school days.

In addition, the youths would be allowed to work four hours a day and 24 hours a week while school is in session, as opposed to the current limit of three hours a day and 18 hours a week. They also would be permitted to work up to 36 hours in an abbreviated school week.

Another proposal would permit 14- and 15-year-old children to work in previously forbidden jobs such as operation, maintenance and repair of power-driven food slicers and grinders, food choppers and bakery mixers, according to the House labor standards subcommittee.

Miller, who had likened the administration's proposals to Fagin, the unscrupulous employer of children in Charles Dickens' Oliver Twist, yesterday welcomed the Labor Department's action as "a substantial victory."